Gastrointestinal (GI) cancers continue to be a significant and challenging clinical problem. Surgical resection is the mainstay for cure of GI malignancies; however, cure can only be achieved if the tumors are localized and have not spread to lymph nodes and distant organs. Increased understanding of the molecular mechanisms controlling GI cancer cell growth is required for the development of novel thera-peutic strategies to be used in combination with surgical resection. GI peptide hormones can stimulate the growth of normal and neoplastic gut tissues. For years, our studies have focused on determining the molecular mechanisms by which the gut peptide, gastrin (G-17), and its cognate receptors, regulate cell growth. Recently, we have discovered a novel splice variant of the CCK-B/gastrin receptor called CCK-BRi4sv that is expressed in colonic and pancreatic cancers, but not the normal tissues. CCK-BRi4sv exhibit distinctly different signaling properties when compared to the previously characterized wild-type CCK-BR (CCK-BRwt) including G-17-independent stimulation of cell growth, regulation of intracellular Ca 2+and subcellular trafficking. Also, we found that mitogen-activated protein kinases (MAPKs) play a key role in CCK-BR-mediated signaling both before and after agonist stimulation. MAPK kinase (MEK) regulates CCK-BRwt sensitivity to G-17 stimulation and mediates the effects of G-17 stimulation on downstream effectors. Finally, we found that CCK-BRi4sv and CCK-BRwt mediate G-17-induction of COX-2 gene expression. Based on our findings, we hypothesize that CCK-BR variants regulate GI cell growth by both agonist-dependent and -independent mechanisms, and that MAPKs play a central role in receptor-mediated regulation of cell growth by modulating the sensitivity of the receptor to agonist stimulation and by acting as downstream effectors of agonist-induced signal transduction. To examine these hypotheses, we have planned experiments with three Specific Aims. Aim 1: To define the mechanisms of CCK-BRwt and CCK-BRi4sv internalization and intracellular receptor trafficking. Aim 2: To define the role of MAPKs in CCK-BRwt- and CCK-BRi4sv-mediated intracellular signal transduction. Aim 3: To determine the effects of CCK-BRi4sv and CCK-BRwt expression on gene expression.